1. Field of the Invention
The present invention relates to nonvolatile memories, and more particularly to a nonvolatile memory comprising a FLOTOX type memory (Floating gate with tunnel oxide) and to a process for fabricating the same.
2. Description of the Related Art
So-called nonvolatile random access memories comprising an EEPROM and a DRAM in combination have recently attracted attention as nonvolatile semiconductor devices which are randomly accessible.
The nonvolatile random access memory (hereinafter referred to as "NV-DRAM") is smaller in cell size than conventional nonvolatile devices comprising an EEPROM and an SRAM in combination and makes it possible to realize a higher packing density.
FIG. 6 shows a typical example of NV-DRAM. With reference to the drawing, the NV-DRAM comprises a semiconductor substrate 1, and transistors formed on the substrate, i.e., a transistor MT of FLOTOX structure constituting an EEPROM, and MOS transistors T1 and T2 constituting a DRAM. In the drawing, indicated at 2a is a tunnel oxide film, at 3 a floating gate, at 4 a select gate, at 5 a recall gate, at 6 a control gate, at 7 a bit line, and at 8 a polysilicon layer serving as a charge storage node for the DRAM. FIG. 7 shows a circuit equivalent to the NV-DRAM.
With the conventional NV-DRAM, the tunnel oxide film is formed by photolithography for tunnel region patterning after ion implantation for forming the source region S of the EEPROM.
Accordingly, the conventional tunnel oxide film is laid out in the active region 9 of the NV-DRAM within an impurity ion implantation pattern 10 for forming the source region as seen in FIG. 2.
Thus, the layout of tunnel oxide film of the conventional NV-DRAM involves a restriction in that the tunnel oxide film is so positioned as to be included within the impurity ion implantation pattern 10. Additionally, since the region of tunnel oxide film is formed by photolithography, the reduction in the size or width thereof is limited, consequently presenting difficulties in providing NV-DRAMs of reduced size with a higher packing density.
Further, the area of the tunnel oxide film depends on the accuracy of microscopic processing technology in photoetching, limiting the reduction in area. At the same time, the conventional fabrication process involves a problem of misalignment of the tunnel oxide region with the impurity diffusion region, limiting scale-down in cell size.